Our results show that postaglandins (PGs) affect a number of metabolic parameters in human embryo lung fibroblasts (HELF), including cyclic AMP (cAMP) levels, cAMP phosphodiesterase activity, substrate uptake and cell replication. Also, that cAMP modulates the growth of HELF. We also showed that the cAMP content, per cell, in late passage ("old") HELF increases and that the response to PGE in terms of cAMP levels alters in late passage HELF. We found that a number of pleiotypic parameters in late passage cells respond similarly to those in early passage cells to growth stimulation with serum, yet these "old" cultures, consequently, undergo only minimal stimulation. Our overall objective is to determine cell aging as it occurs at the level of the plasma membrane. The prostaglandins and the cAMP system are two parameters functioning and interacting within this organelle. The participation of both of these metabolic systems in aging is under further investigation by us.